The invention relates to a method of diagnosis of mycobacterial infection, particularly Mycobacterium tuberculosis infection. It also relates to peptides and a kit which can be used to carry out the diagnostic method.
Current diagnostic tests for tuberculosis disease are either slow or unreliable. Tests that rely on the identification of the mycobacterium which causes tuberculosis are slow because culturing of the mycobacterium can take up to 8 weeks. In some cases it proves impossible to culture the bacteria. In addition the obtaining of samples to detect the presence of the mycobacterium often requires invasive procedures.
An alternative test is the tuberculin skin test (TST) or Mantoux test which is based on the detection of a delayed type hypersensitivity (DTH) response to an intradermal administration of a Purified Protein Derivative of the mycobacterium. Although this test takes less time than tests which rely on identification of the mycobacterium, it is less reliable because of the widespread use of BCG as a vaccine against tuberculosis. BCG is closely related to M. tuberculosis and therefore individuals who have been vaccinated with BCG can react positively to a TST. In addition a large proportion of people with active tuberculosis are not detected by a TST because of cutaneous immune anergy. Thus TST has a low specificity and sensitivity.
Using an assay which detects release of IFN-γ from T cells the inventors have found 8 peptides from the ESAT-6 protein of M. tuberculosis which are recognised by the T cells of a high proportion of patients with tuberculosis, and in particular the peptide represented by SEQ ID NO: 1 is recognised by 57% of patients tested and 68% of healthy contacts tested. These contacts have been exposed to open pulmonary tuberculosis. The inventors have combined these peptides into a panel of peptides which when used together in a diagnostic test provide a specificity of 91.5%, and a sensitivity of 96%. The inventors have also found three other peptides from ESAT-6 which are recognised by the T cells of patients with tuberculosis which can be used to increase the sensitivity of the diagnostic test.
Advantageously BCG does not have the ESAT-6 gene and therefore unlike previous tests, including TST, the diagnostic test can distinguish between patients with tuberculosis and patients who have been vaccinated with BCG.
Brandt et al. (1996), Journal of Immunology, 157, 3527-33 discloses epitopes from ESAT-6 which are recognised by mice. However it is not possible to predict based on the epitopes which are recognised in mice which epitopes will be recognised in humans. As well as other differences in epitope processing, presentation and recognition mice have different MHC molecules from humans, and thus are expected to recognise different epitopes from humans. This is demonstrated by the fact that Brandt et al find the recognition of epitopes in mice which are not found to be recognised in humans by the present inventors.